The present invention relates to neutron detectors, and more particularly relates to an array of solid state neutron sensors capable of measuring a wide range of neutron fluxes generated by nuclear power reactors and the like.
Neutron detection is conventionally accomplished by using a detector of ionizing radiation and employing a conversion nuclear reaction whereby the neutron produces a charge particle product. The first neutron detectors were gas proportional counters which detected ionization produced by the highly charged fission fragments produced in neutron induced fission of 235 U.
In pressurized water reactors, three types of gas proportional counters are typically used to provide coverage over the full range of neutron fluxes that are normally experienced during reactor operation. For the low neutron fluxes at reactor startup, a BF3 proportional counter is used to record pulses from the10 B(n,xcex1) reaction. This detector is operated in the pulse-counting mode until the neutron flux reaches the 104 to 105 cmxe2x88x922-secondsxe2x88x921 range, corresponding to 105 to 106 counts per second in the detector. At these count rates, linearity in count rate as a function of neutron flux begins to deteriorate due to the pulse pile-up, and at about 106 counts per second, individual pulses cannot be distinguished. The neutron flux range from startup to about 104cmxe2x88x922-secondxe2x88x92is referred to as the Source Range. A second type of detector, a boron-lined ionization chamber, is used to monitor neutron flux above 10 3cmxe2x88x921-secondxe2x88x921. Because of the inherently large count rate, this counter is operated in the current mode, i.e., individual pulses are summed to form a current which is monitored. However, although the current generated by neutron counts is proportional to neutron flux and to reactor power, current is also generated by gamma ray interactions with the detector. Therefore, gamma compensation must be used, because gamma ray intensity is not proportional to reactor power in the low power range. A second detector, without a boron liner, which was used in the first detector to generate neutron-induced particles for counting, is used to determine current generated by gammas only and this current is subtracted from the observed current for the boron-lined first detector. This pair of ionization chambers is referred to as a compensated ion chamber (CIC). CICs are used to monitor reactor power from neutron fluxes of about 103cmxe2x88x922-secondxe2x88x921 to above 1010cmxe2x88x922-secondxe2x88x921, which is close to full reactor power. This range of neutron fluxes is referred to as the Intermediate Range, and the BF3 and CIC power monitors are referred to as the Source Range and Intermediate Range detectors, respectively. For neutron fluxes higher than about 107cmxe2x88x922-secondxe2x88x921 , gamma ray intensity becomes proportional to power, and gamma compensation is no longer necessary. Boron-lined ion chambers, without gamma compensation, are used from 107cmxe2x88x922-secondxe2x88x921 to 2.5xc3x971010cmxe2x88x922-secondxe2x88x92, the range of neutron fluxes referred to as the Power Range. The Source, Intermediate and Power Ranges are indicated in FIG. 1, which shows the thermal neutron flux in neutrons/cm2/second at the detector location over the Source Range 10, Intermediate Range 12 and Power Range 14. The Source Range is shown in counts per second and the Intermediate and Power Ranges are shown in amperes.
In order to monitor power from startup to full power, data from all three types of detectors are needed. During the transition from the high end of the Source Range into the lower end of the Intermediate Range, the responses from the Source Range and Intermediate Range detectors must be matched. This matching can be particularly difficult because two inherently different types of detectors are being used. A pulse mode Source Range detector must be matched to a current mode, gamma compensated Intermediate Range detector. Difficulties in matching these responses when changing power can result in operating delays and, in extreme cases, in reactor trips. To further complicate the matter, due to the extreme sensitivity of the Source Range detector, to avoid early failure, the Source Range is turned off during Power Range operation. It has not been uncommon for the Source Range detector to fail when an attempt is made to reactivate the detector upon power down of the reactor.
An alternative to using a gas-filled detector is to use a semiconductor or solid state detector. Conventional semiconductor neutron detectors consist of a silicon surface barrier detector with a layer of boron, lithium or fissionable material adjacent to the active volume of the detector. One such solid state neutron detector using silicon semiconductor is disclosed in U.S. Pat. No. 3,227,876.
A problem with prior art neutron detectors is sensitivity of the detector to non-neutronic components of the radiation field, particularly gamma ray sensitivity. Gas-filled detectors are favored in nuclear reactor applications because low density gases are inherently inefficient detectors for gamma rays which deposit their energy over large volumes. Solid state detectors, on the other hand, are more sensitive to gamma rays because of their higher electron density.
Solid state semiconductor detectors are candidates for replacement of conventional gas filled detectors, but they have not found widespread use in the nuclear industry because of problems associated with background signal and deterioration of detector performance during operation in intense, hostile radiation environments.
U.S. Pat. No. 5,726,453, issued Mar. 10, 1998 and U.S. Pat. No. 5,940,460, issued Aug. 17, 1999, describe a radiation resistant solid state neutron detector that can discriminate between neutron and gamma responses on the basis of pulse height. Based on these two properties displayed by, but not necessarily limited to, silicon carbide (SiC) neutron detectors, an improved solid state neutron detector is desired that can operate over the wide range of reactor operation. Furthermore, it is an objective of this invention to provide such a detector that can operate over the entire range of operation of a reactor in the pulse mode. Additionally, it is an object of this invention to provide a detector that can function over the entire range of operation of the nuclear reactor employing a single type of electronics train to process the detector output.
These and other objects are achieved by employing a detector having at least one array of sensors, each sensor having a semiconductor active region for generating an electronic output signal in response to charged particles emitted from a converter layer in response to the neutron emissions from a neutron source such as a reactor core. The electronic output is processed by a single electronics train for each detector for processing the electronics output signals of each of the arrays of sensors associated with that detector to provide an output indicative of the power of the source. Means are also provided for changing the sensitivity to neutron emissions of the detector when the electronic output pulse count exceeds or drops below pre-selected rates, e.g., the count rate exceeds the rate the electronics train can discriminate counts from or the count rate is too slow to provide meaningful information.
In one embodiment, the detector includes at least two arrays of sensors with each array having a different sensitivity. The electronics train then switches between arrays when the pre-selected rates of count are detected.
In another preferred embodiment, the means for changing the sensitivity of the detectors changes the converter layer composition either by changing the depth or makeup of the converter material.
An additional feature of the invention is that during full power operation, it alters the converter material associated with sensors whose outputs are not being monitored so the corresponding semiconductor active region is not bombarded with charged particles. Additionally, the detector of this invention can be employed to provide a more accurate display of the axial power distribution of a reactor core than current ex-core detectors.